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North Korea claimed to have achieved the “next level” of “nuclear might” yesterday after conducting the underground explosion of a “hydrogen bomb”.

This was the country’s fourth nuclear test since 2006, but the first allegedly to involve a hydrogen weapon – also known as thermonuclear device.

Bombs of this kind, which rely on nuclear fusion, release far more destructive power than other weapons based on a fission reaction. If North Korea’s claim was true, the country would have joined the ranks of the world’s thermonuclear powers.

A joyful announcer on state television said the “perfectly successful” test had brought the country’s “nuclear might to the next level".

North Koreans celebrate as they watch a news broadcast on a video screen outside Pyongyang Railway Station

But experts were sceptical, pointing out that the earthquake caused by the detonation at the Punggye-ri test site measured 5.1 on the Richter scale. This was no stronger than the tremor recorded during the last nuclear test in 2013.

America also cast doubt on the boast. "The initial analysis is not consistent with the claim the regime has made of a successful hydrogen bomb test," said Josh Earnest, the White House spokesman.

None the less, the trial was viewed as further evidence of the steady expansion of North Korea’s nuclear ambitions. Mr Kim’s regime is now believed to possess up to 20 nuclear weapons – about half of which are loaded onto ballistic missiles. Last year, North Korea tested a submarine-launched missile and declared its intention of mastering the technology for thermonuclear weapons.

But South Korea, which is routinely threatened with invasion or even nuclear destruction by its neighbour, denounced the latest test as a “grave provocation”. President Park Geun-hye called an emergency meeting of South Korea’s National Security Council, accusing the North of a “direct challenge to world peace and stability”.

Despite being North Korea’s only ally, China joined in the condemnation. The foreign ministry in Beijing said that China “firmly opposes” the nuclear test, adding that North Korea should “stop taking any actions that would make the situation worse”.

The nuclear test breaches a series of UN resolutions. Last night, the Security Council was meeting in emergency session to consider whether to tighten economic sanctions. But North Korea’s economy is bankrupt in any event and China generally opposes tougher measures.

North Korea’s apparent aim is to build an arsenal of thermonuclear weapons that could eventually be delivered on long range missiles. The Taepodong-2 missile, currently in development, could fly as far as the United States.

Satellite photo of North Korea's nuclear test site Photo: AP

Experts cautioned that it was too early to say whether yesterday’s test marked a significant advance towards these goals. The “yield” – or explosive power - of the device was believed to be similar to the six kilotons recorded during the last trial.

“One thing that is clear is they are making progress across their nuclear arsenal,” said Matthew Cottee, a non-proliferation expert at the International Institute for Strategic Studies. “Both the nuclear programme the missile programme are continuing without any significant control by the international community.”

Air particles, satellite images and seismic data will all be analysed to learn more about this test – and establish exactly what device was used. But Mr Cottee added: “Underground testing makes it a lot easier to cover their tracks and make it harder for the international community to find out what they’re doing.”

China joins in condemnation of North Korea

All 15 members of the UN Security Council have joined in condemning the nuclear test and calling it "a clear threat to international peace and security".

"The members of the Security Council ... recalled that they have previously expressed their determination to take further significant measures in the event of another DPRK (North Korea) nuclear test," Elbio Rosselli, Uruguay's UN ambassador and president of the council this month, told reporters.

"In line with this commitment and the gravity of this violation, the members of the Security Council will begin to work immediately on such measures in a new Security Council resolution," he said, noting that the test was "a clear violation of Security Council resolutions."

"Difficult, time consuming" task to verify if test was hydrogen bomb

The hunt is now on among nuclear experts to track down airborne radioactive particles which can provide crucial clues as to whether North Korea really has tested a hydrogen bomb.

According to Reuters, the location of the quake, near a known North Korean nuclear test site, and its seismic characteristics led experts to quickly conclude that North Korea had probably conducted a fourth nuclear test. Pyongyang then announced it had done so.

But it is the presence of radioactive particles which should help investigators decide whether North Korea's is as dangerous as feared.

Until those particles are analysed it will be very difficult to confirm either way, according to investigators.

Whereas atomic bombs such as those already tested by North Korea in 2003 and 2009 are powerful weapons, they are nowhere near as devastating as hydrogen bombs.

But this has not stopped the international community from being gripped with alarm over the test, which shows North Korea is another step closer to becoming one of the leading nuclear threats on earth.

Japan has already dispatched H4 training planes to investigate whether the test has unleashed nuclear material in the atmosphere, though initial reports say this is unlikely.

Meanwhile, in Pyongyang, North Koreans were seen "celebrating" the launch as they gathered around TV screens at the capital's rail station - many of them huddled in winter coats to keep out the bitter cold.

It was "deeply troubling" and "profoundly destabilizing for regional security," he said.

"This test once again violates numerous Security Council resolutions despite the united call by the international community to cease such activities...It is also a grave contravention of the international norm against nuclear testing. I demand the DPRK (North Korea) cease any further nuclear activities and meet its obligations for verifiable denuclearization.

Graphic: The world's nuclear armed powers

North Korea 'thinks it can survive nuclear counter strike by hiding in tunnels'

Mr Everard added that North Korea probably believed it could survive an all-out nuclear war as it has a complex network of defensive tunnels:

Moreover, North Korea has an elaborate system of defensive tunnels – like Tora Bora in Afghanistan, but much more extensive. If the regime believes that it could retreat to these tunnels and survive a nuclear counter-strike then it will be less reluctant to use its own nuclear weapons.

Kim Jong-un visited the recently refurbished Phyongchon Revolutionary Site in central Pyongyang. The museum regularly appears in official documentaries about revolutionary history and on the DPRK?s military industrial complex. Photo: KCNA/Reuters

"It's dangerous to assume North Korea won't ever use a nuclear weapon"

John Everard, the former British ambassador to North Korea, writes for The Telegraph today on the dangers of being complacent about Kim Jong-un.

Was North Korea's test on January 6 really a hydrogen bomb? If it was, would it be able to deliver such a bomb to a target?

Even if it has the technical capability does it have the intent – or would North Korea's leaders assess that a nuclear attack would invite devastating nuclear retaliation and so the end of their regime (and perhaps of North Korea as a state)?

It is dangerous to assume that North Korea would never really use a nuclear weapon. Although we may think that this would mean the end of the regime it is not clear whether the regime thinks this. In the strange, closed world of North Korea it is quite possible that the leadership has convinced itself that craven foreigners would not dare to counter-attack if it used a nuclear device.

All smiles in North Korea as world awaits UN Security Council reaction

This sligthtly bizarre image, snapped on the Korean border, shows one North Korean soldier looking rather julibant. And the mood in Pyongyang - as relayed to us by the state's propoganda machine of course - has been one of pure celebration today.

A North Korean soldier smiles near the North Korean town of Sinuiju, as seen from the opposite Chinese border city of Dandong,

North Korean people celebrate the success of the first hydrogen bomb test in Pyongyang.

Trump: If China doesn't solve North Korea, we should make trade "difficult" for them

Speaking to Fox News, Trump said: "China has total control...They have total control over North Korea, and China should solve that problem. And if they don't solve the problem, we should make trade very difficult for China."

He adds that Kim Jong-un is a "madman" and that "nobody ever talks to him except Dennis Rodman."

Did North Korea's row with China begin over girl band hand picked by Kim Jong-un?

North Korea's nuclear test will be seen as nothing short of insolence by Beijing, who have joined today's chorus of international condemnation.

The move suggests a rift in relations between Kim Jong-un and his most powerful ally in the east - which is odd considering how well the two countries appeared to be getting on recently.

As already reported, Kim was photographed beaming with pride when he met a high-ranking Chinese official last October.

But today's "slap in the face" to China may well have been foreshadowed by an incident last December, when North Korean girl band Moranbong sudennly cancelled a tour in China and left the country abruptly.

The group was handpicked by the regime leader himself and are known to play songs which exhult his charming smile and "warm heart."

It remains unclear why the band suddenly cancelled the show in Beijing - it merely cited "a reason" in an announcement on its Facebook page. But it was speculated Kim pulled the group out of China as a means of expressing his annoyance with Beijing.

Hydrogen bomb test "a slap in the face to China"

Mike Chinoy, author of Meltdown: The Inside Story of the North Korean Nuclear Crisis, has appeared on CNN.

He said:

Beijing has got to be absolutely furious.

There was a period when the Chinese were really angry with the Koreans but some months ago there were signs of a clear shift in the Chinese approach. They sent a senior official, Liu Yunshan, to a big military parade in October, standing right next to Kim Jong Un. It was all smiles.

Video: View from Pyongyang as North Koreans celebrate test "success"

Nato urges North Korea to abandon nuclear programme

NATO head Jens Stoltenberg wants North Korea to drop its nuclear weapons setup - but as widely reported Kim Jong-un's nuclear menace is his main tool to justify his position as supreme leader of North Korea.

The nuclear weapons test announced by North Korea undermines regional and international security, and is in clear breach of UN Security Council resolutions.

North Korea should abandon nuclear weapons and existing nuclear and ballistic missile programmes in a complete, verifiable and irreversible manner, and engage in credible and authentic talks on denuclearisation.

What we know about North Korea's "hydrogen bomb"

If confirmed, North Korea's claim to have tested a small hydrogen bomb would put it in a small group of countries.

Nuclear arsenals across the world are usually split into two parts, atomic bombs (known as a-bombs) and more powerful hydrogen bombs.

The Nagasaki and Hiroshima bombs, for example, were a-bombs.

Early estimates at the power of North Korea's bomb puts it at six kilotons, which is roughly the same level of force as the bombs used in previous tests in 2003 and 2009. Hiroshima's power was roughly 15 kilotons.

Six kilotons is far smaller than any blast produced by a hydrogen device, prompting South Korea to suggest this bomb may have used enhanced nuclear fission techniques but is not a full scale nuclear bomb.

North Koreans celebrate as they watch a news broadcast on a video screen outside Pyongyang Railway Station

Russia says test "a new step on path to nuclear weapons"

Russia's foreign ministry has also reacted to the test. A spokesman said:

On January 6 the North Korean government announced a successful test of a hydrogen bomb. Russia is studying this statement closely and analysing technical information. If such a test can be confirmed it would represent another step by Pyongyang towards development of nuclear weapons, which would be a grave violation of international law and UN Security Council resolutions. Such actions are fraught with the danger of aggravating tensions on the Korean peninsula, which is already characterised by high potential for political or military confrontation.

EU says test "grave violation" of UN resolutions

If confirmed, this action would represent a grave violation of (North Korea's) international obligations not to produce or test nuclear weapons, as determined by several United Nations Security Council Resolutions, and a threat to the peace and security of the entire northeast Asia region.

Full statement from Philip Hammond on hydrogen bomb claims

Neil Connor in Beijing has been in touch with the Foreign Secretary's office.

Foreign Secretary Philip Hammond said: "If a nuclear device has been detonated by North Korea, this is a grave breach of UN Security Council resolutions and a provocation which I condemn without reservation.

"It underlines the very real threat that North Korea represents to regional and international security.

"We will be working with other UN Security Council members to ensure the international community responds urgently and decisively to this latest activity."

Mr Hammond added: "I have discussed this matter today in Beijing with my Chinese counterpart, State Councillor Yang Jiechi, and we have agreed to work with other members of the UN Security Council towards a robust international response."

Mr Hammond is also due to speak about these developments today with his Japanese and South Korean counterparts. He will be visiting Tokyo for high level meetings later this week.

Kim Jong-un's handwritten test letter

We mentioned earlier that Kim Jong-un provided two handwritten letters which were broadcast on state television during the announcement of the test. Here's the first one.

It reads: "I approve the test to safeguard the Party Centre. Carry it out on January 6, 2016."

A message of North Korean leader Kim Jong-Un for its first hydrogen bomb test. The Korean letters read that "I approve the test to safeguard the Party Centre. Carry it out on January 6, 2016. Photo: AFP

The second is a little more colourful, jubilant, even.

Let's begin the year of 2016 -- a glorious and victorious year when the historic seventh conference of the Workers' Party of Korea will be held -- with a thrilling sound of the first hydrogen bomb explosion, so that the whole world will look up to our socialist, nuclear-armed republic of Juche and the great Workers' Party of Korea!

A message of North Korean leader Kim Jong-Un for its first hydrogen bomb test. The Korean letters read that "Let's begin the year of 2016 -- a glorious and victorious year when the historic seventh conference of the Workers' Party of Korea will be held -- with a thrilling sound of the first hydrogen bomb explosion, so that the whole world will look up to our socialist, nuclear-armed republic of Juche and the great Workers' Party of Korea! Photo: AFP

I have discussed this matter today in Beijing with my Chinese counterpart, State Councillor Yang Jiechi, and we have agreed to work with other members of the UN Security Council towards a robust international response. Hammond said in a statement.

If North Korean H-bomb test reports are true, it is a grave breach of #UNSC resolutions & a provocation which I condemn without reservation

Kim Jong-un announced the hydrogen bomb test in a handwritten note that was broadcast on state television, it has emerged.

"Let's begin the year of 2016 ... with the thrilling sound of our first hydrogen bomb explosion, so that the whole world will look up to our socialist, nuclear-armed republic and the great Workers' Party of Korea!" Kim wrote in a message next to his signature.

The news was broadcast on state television, which also showed a copy of Kim's initial signed order dated December 15.

The television also showed a second order dated January 3 in which Kim signed off his final approval for the test to be conducted on January 6, AFP reported.

North Korean leader Kim Jung-un guides the test firing of a rocket in this photo released by North Korea's Korean Central News Agency in Pyongyang. North Korea is "likely" to have conducted a nuclear test on January 6, 2016 that caused an earthquake near a known testing site in the isolated country, the South Korean and Japanese governments said. Photo: REUTERS/KCNA

Good morning

Chinese border towns have been evacuated, UN officials are queuing up to condemn the test and there is a general sense of acopalyptic anxiety as the world awaits confirmation as to whether North Korea now poses a genuine nuclear threat.

If the regime's claims are true then they will raise massive concerns about the hermit state's forbidden nuclear programme.

"The republic's first hydrogen bomb test has been successfully performed at 10:00 am (0330 GMT)," North Korean state television announced.

"With the perfect success of our historic H-bomb, we have joined the rank of advanced nuclear states," it said, adding that the test was of a miniaturised device.

Even China, North Korea's key diplomatic protector, said earlier today that it "firmly opposes" Pyongyang's nuclear test, adding it was carried out "irrespective of the international community's opposition".

Until now, most experts had assumed Pyongyang was years from developing a thermonuclear bomb.

North Korea times nuclear test to perfection

By Julian Ryall in Tokyo

The underground nuclear test conducted by North Korea on Wednesday may have been billed as an early birthday present for Kim Jong-un, the North Korean leader, but analysts believe the regime has timed its latest show of force carefully.

"North Korea has announced that it will hold its first party congress in 35 years in May and Mr Kim may not feel that he has done enough to cement his position, so showing that he now has a hydrogen bomb is calculated to significantly boost his standing," Toshimitsu Shigemura, a professor at Tokyo's Waseda University and an authority on the North Korean leadership, told The Telegraph.

"I also believe that by carrying out the test now, Mr Kim has three months to soothe relations with China, which is still his most important ally," he added.

Beijing is additionally concerned about unrest at home over the uncertain economic situation and will shy away from any course of action that might provoke further domestic disquiet.

Another consideration will have been Western governments' preoccupation with events in the Middle East, particularly given the escalating crisis between Saudi Arabia and Iran.

Given other global flashpoints, analysts believe the international community may be unable to muster much of a response and that Mr Kim may have calculated wisely.

"The international community will condemn these actions, but we have no effective way of counteracting this development," he said. "Even China, which has acted as a stabilising force in the past, does not have so many cards to play."

Prof Shigemura expects Pyongyang to be more conciliatory again later in the year, after the dust from the test has settled and Mr Kim has been able to demonstrate his mastery of his enemies at the Party Congress.

That would echo the tried-and-tested approach by the regime over several decades that has promised detente and cooperation but served only to buy time for Pyongyang to develop nuclear weapons and the long-range missiles with which to deliver to them.

"And President [Barack] Obama is entering his final year in office and many previous US leaders have tried to make progress on the Korea question as their terms come to an end; Mr Kim may very well be expecting the president to do something similar and offer talks."

North Korea's announcement of 'successful' nuclear test on state TV

Device had 'small amount of hydrogen'

South Korea's defence ministry says a small amount of hydrogen may have been added to the device that North Korea tested, Reuters reports.

"We will find out after closely analysing it but we understand a small amount of hydrogen may have been added to the fourth nuclear test," ministry spokesman Kim Min- Seok said, according to the MoneyToday online news service.

The claim was questioned by international experts and there was continued scepticism over Wednesday's test announcement.

"The seismic data that's been received indicates that the explosion is probably significantly below what one would expect from an H-bomb test," said Crispin Rovere, Australian nuclear policy and arms control specialist.

Bruce Bennett, a senior defence analyst with the Rand Corporation, was equally unconvinced.

"This weapon was probably the size of the US Hiroshima bomb but this was not a hydrogen bomb," Bennett told the BBC.

"The bang they should have gotten would have been 10 times greater than what they got," he added.

The location of the 'earthquake' as observed by the US Geological Survey Photo: USGS

The test, which came just two days before Kim Jong-Un's birthday, was initially detected by international seismology centres as a 5.1-magnitude tremor next to the North's main Punggye-ri nuclear test site in the northeast of the country.

Whether an H-bomb or not, it was North Korea's fourth nuclear test and marked a striking act of defiance that flew in the face of enemies and allies alike who have warned Pyongyang it would pay a steep price for moving forward with its nuclear weapons programme.

The three previous tests in 2006, 2009 and 2013 triggered waves of UN sanctions. Their failure to prevent a fourth detonation will place the Security Council under intense pressure to take more drastic action this time around when it meets later on Wednesday.

If this test is confirmed, it will be just the latest example of the failed Obama-Clinton foreign policy. (3/5)

It throws down a particular challenge to Barack Obama, who, during a visit to South Korea in 2014, lashed North Korea as a "pariah state" and vowed sanctions with "more bite" if Pyongyang went ahead with another test.

Ned Price, White House National Security Council spokesman, said he could not confirm the H-bomb claim, but promised the US would "respond appropriately to any and all North Korean provocations".

South Koreans at Seoul Railway Station watch a television broadcast reporting North Korea's Hydrogen bomb test Photo: Chung Sung-jun/Getty Images

South Korea "strongly condemned" the test and warned Pyongyang that it would be made to "pay the price" for ignoring international opinion.

Shinzo Abe, Japanese Prime Minister, called it a "serious threat" to Japan and a "grave challenge" to nuclear non-proliferation efforts.

Seoul watches

Monitoring body says test is threat to peace

The head of an international body set up to monitor a planned ban on nuclear testing condemned North Korea's fourth nuclear test on Wednesday and called it a "wake-up call" for the international community.

North Korea said it had successfully conducted a test of a miniaturised hydrogen nuclear device, which would mark a significant advance in the isolated state's technological capability.

Lassina Zerbo, head of the Comprehensive Nuclear-Test-Ban Treaty Organisation, said:

This act constitutes a breach of the universally accepted norm against nuclear testing. It is also a grave threat to international peace and security.

Crisis meeting in Tokyo

Celebrations in Pyongyang

Crowds dressed in thick winter coats have gathered outside a large video screen near a Pyongyang train station to cheer and take video and photos on their mobile phones of the state TV anchor announcing the country had carried out a nuclear test, according to the Associated Press.

Some people raised their hands and applauded. Many smiled and cheered.

White House calls on North Korea to abide by its international commitments

The White House said late on Tuesday that it could not confirm North Korea's claims that it had successfully conducted a test of a miniaturised hydrogen nuclear device, but it will continue to monitor the situation.

"While we cannot confirm these claims at this time, we condemn any violation of UNSC Resolutions and again call on North Korea to abide by its international obligations and commitments," said Ned Price, White House National Security Council spokesman.

Mr Price said the United States will continue to "protect and defend our allies in the region," and will "respond appropriately to any and all North Korean provocations."

'Days before test can be confirmed'

UN Security Council will discuss North Korea

The world is starting to react.

The UN Security Council is expected to meet at 11:00 on Wednesday in New York (16:00 GMT) while a White House official says the administration is aware of seismic activity in vicinity of a known nuclear test facility.

So was it a hydrogen bomb?

A North Korean newsreader speaks during a broadcast on North Korean KRT

Although the state news reader announced the successful testing of a hydrogen bomb, analysts will now try to detect any gases leaking from the underground blast in order to determine exactly what sort of bomb it was.

Hydrogen bombs release more energy - and less radioactive fallout - by using a fusion reaction rather than a fission chain reaction of an atomic bomb. Lighter elements are fused into heavier ones, releasing huge amounts of energy in the process. However, it needs to be triggered by fission reaction in a two-step process.

South Korea steps up monitoring

South Korea's defence ministry says that the country's armed forces are stepping up their monitoring of North Korea after the isolated country said it successfully conducted a test of a miniaturised nuclear device.

Separately, South Korea's Yonhap News agency said President Park Geun-hye will preside over a national security council meeting at 04:30 GMT.

Shinzo Abe promises tough Japanese reponse

Shinzo Abe, Japanese prime minister, said on Wednesday the nation would make a firm response to North Korea's challenge against nuclear non-proliferation, calling its latest nuclear test a threat to Japan's security.

Mr Abe, speaking to reporters, said Japan absolutely could not tolerate North Korea's nuclear testing. He added it was a "grave challenge" to no-proliferation efforts.

The nuclear test that was carried out by North Korea is a serious threat to the safety of our nation and we absolutely cannot tolerate this.

North Korea says it needs H bomb to defend against US aggression

The US has gathered forces hostile to DPRK and raised a slanderous human rights issue to hinder DPRK’s improvement … It is just to have H-bomb as self-defence against the US having numerous and humongous nuclear weapons. The DPRK’s fate must not be protected by any forces but DPRK itself.

North Korea claims to have tested hydrogen bomb

North Korea said Wednesday it had carried out a "successful" hydrogen bomb test, a claim that - if true - massively raises the stakes over the hermit state's banned nuclear programme.

In a surprise announcement Pyongyang said it had carried out a hydrogen blast.

"The republic's first hydrogen bomb test has been successfully performed at 10:00 am on January 6, 2016, based on the strategic determination of the Workers' Party," a state television news reader said.

A hydrogen, or thermonuclear device, uses fusion in a chain reaction that results in a far more powerful explosion than the fission blast generated by uranium or plutonium alone.

Pentagon is investigating

The US Defence Department is "looking into reports of a possible seismic event near North Korea's nuclear facilities.

South Korea's meteorological agency said earlier that North Korea is "likely" to have conducted a nuclear test that caused an earthquake near a known testing site in the isolated country. The US official said:

We are looking into the reports of a possible seismic event near North Korean nuclear facilities.

Crucial year in north-south relations

A missile carrier taking part in the military parade at the Kim Il-sung Square in Pyongyang on April 15, 2012 Photo: AP

More from Julian Ryall in Tokyo:

What appears to have been North Korea's fourth nuclear test went ahead a day after Park Geun-hye, the South Korean president, called on her cabinet to ensure that the nation maintains its readiness to deal with any security challenges.

Addressing her cabinet in its first meeting of the new year, Mrs Park said she would continue with efforts to build bridges with the regime in Pyongyang, but called for the government to remain "vigilant".

"This year, we are at a very critical turning point in terms of diplomacy and security and must therefore stay vigilant at all times", Mrs Park said, adding that a "solid and reliable national defence is the paramount foundation".

If North Korea confirms it has carried out a new nuclear test, efforts by the South to improve cross-border ties will inevitably return to the back-burner

Epicentre of "quake" close to known test site

The US Geological Survey said the epicentre of the quake - detected at 10:00 am Pyongyang time (01:30 GMT) - was in the northeast of the country, some 30 miles northwest of Kilju city, placing it right next to the Punggye-ri nuclear test site.

Waveforms similar to previous tests

Sabre-rattling yesterday

Julian Ryall, our correspondent in Tokyo, has this:

The suspected blast comes a day after North Korea proclaimed it had the legitimate right to a nuclear weapons programme, with Pyongyang insisting it had no choice but to strengthen its defences with nuclear weapons in the face of a "hostile policy" pursued by the United States. In an editorial released on Tuesday, the Korea Central New Agency said:

We deserve to hold nuclear weapons and ceaselessly strengthen our 'byeongjin' policy to counter nuclear threats by the US

A test would further North Korea's international isolation by prompting a push for new, tougher sanctions at the United Nations and worsening Pyongyang's already bad ties with Washington and its neighbours.

Pyongyang is thought to have a handful of crude nuclear weapons. The United States and its allies worry about North Korean nuclear tests because each new blast brings the country closer to perfecting its nuclear arsenal.

Suspected man-made explosion reported in North Korea

Hello and welcome to our live coverage of the news that there has been a suspected explosion detected in North Korea. It had initially been recorded as an earthquake measuring 5.1 magnitude. South Korea has said it is monitoring the situation closely.